Malaria constitutes one of the most devastating global health problems in human history. Infection with malarial parasites affects more than 207 million people annually, killing ˜627,000 children. (World Malaria Report 2013). More than 85% of malaria cases and 90% of malaria deaths occur in sub-Saharan Africa, mainly in young children (ie, those younger than 5 years).
Malaria is a protozoan disease, caused by Plasmodium species, which are transmitted by Anopheles mosquitoes. There are more than 400 anopheline species known to date, however, only around 25 are good vectors for the transmittal of the disease (Sinka M E, et al. Parasit Vectors 2012; 5: 69). About 200 Plasmodium species are known (cf. Martinsen et al. Mol Phylogenet Evol. 2008 April; 47(1):261-73), out of which only five species of the genus cause all malarial infections in human beings. Most cases are caused by either Plasmodium falciparum or Plasmodium vivax, but human infections can also be caused by Plasmodium ovale, Plasmodium malariae, and, in parts of southeast Asia, the monkey malaria Plasmodium knowlesi. Almost all deaths are caused by P. falciparum malaria. Malaria in pregnancy, which is caused by both P. falciparum and P. vivax causes indirect mortality from abortion and intrauterine growth retardation, which increases infant mortality (White et al., (2014) Lancet; 383: 723-35).
The pathogenesis of malaria is multifactorial, and serious sequalae can result from three primary pathophysiological events: (i) red blood cell destruction; (ii) adhesion of infected erythrocytes to the capillary veins; and (iii) an excessive pro-inflammatory response. Excessive pro-inflammatory response is responsible for sepsis-like signs and symptoms such as rigors, headache, chills, spiking fever, sweating, vasodilatation and hypoglycemia. (Clark et al. Malaria Journal 5 (2006); Stevenson et al. Nat. Rev. Immunol. 4:169-180 (2004); Schofield et al. Nature Reviews Immunology 5:722-735 (2005)). Cerebral malaria is a severe neurological complication of malarial infection and is a major cause of acute non-traumatic encephalopathy in tropical countries. (Idro et al. Lancet Neurol. 4: 827-840 (2005)).
The life cycle of P. falciparum begins with inoculation of motile sporozoites into the dermis of a human by an anophiline mosquito. Sporozoites then travel to the liver, where the sporozoites invade hepatocytes and then multiply. After about a week, the liver schizonts burst, releasing a large number of merozoites into the bloodstream. The merozoites in turn invade erythrocytes and begin the asexual cycle of P. falciparum. Once inside the erythrocyte the merozoites develop inside parasitophorous vacuoles, undergoing several biochemical and morphological changes that can be identified by three stages known as ring, trophozoite and schizont. Following proliferation in the erythrocytes, the infected erythrocytes rupture and release merozoites allowing the continuity of the intraerythrocytic cycle (Bannister et al. (2000) Parasitol Today 16: 427-433).
The illness starts when total asexual parasite numbers in the circulation reach roughly 100 million. Some parasites develop into sexual forms (gametocytes). Gametocytes are taken up by a feeding anopheline mosquito and reproduce sexually, forming an ookinete and then an oocyst in the mosquito gut. The oocyst bursts and liberates sporozoites, which migrate to the salivary glands to await inoculation at the next blood feed. Typically, the entire cycle roughly takes about 1 month.
Recently, it was proven that the sporozoites injected first cross through the dermis and only a few of them migrate into the capillary vessels, while others migrate into lymph vessels and give rise to exoerythrocytic forms, which were unknown until recently. The exoerythrocytic forms may have an important influence on host immunological system (Amino R, et al. (2006) Nat Med 12: 220-224).
In the bloodstream some parasites develop into gametocytes, which are the infectious form of P. falciparum for its vector mosquito in which the sexual cycle occurs. Once in the mosquito the gametocytes mature in the mosquito bowel, a process which is also referred to as gametogenesis. Maturation is followed by fertilization with the union of male and female gametes originating in the formation of a zygote. The zygotes then migrate and adhere to the bowel epithelium, where they develop into an oocyst. Upon rupture of an oocyst, sporozoites are released which then migrate to the salivary gland of the mosquito and are released during mosquito feeding (Ghosh A, et al. (2000) Parasitol Today 16: 196-201).
Besides the great variety of parasite forms in the host and vector mosquito, a noticeable feature of the life cycle of several species of Plasmodium is its synchronization and periodicity. Such distinguished periodicity in formation of gametocytes, the sexual forms of parasite, have been observed since the beginning of last century, and all research done with several species of Plasmodium show the existence of a gametocyte production peak at night, every 24 hours, usually at the same time of mosquito feeding. In this way, the circadian rhythm of the gametocytes is likely to be an important adaptation for maintenance of parasite sexual cycle in the vector mosquito (Garcia C R S. et al. (2001) J Biol Rhythms 16: 436-443). Until now the signal responsible for the induction of gametocytes formation in the vertebrate host bloodstream remains elusive.
The high degree of synchronization of intraerythrocytic asexual forms results in recurring fever attacks and shivers, always in periods of time multiple of 24 hours, coinciding with a practically simultaneously release of billion of merozoites into the bloodstream. Blood-stage infection can persist for months or years or for decades in cases of P. malariae infections when untreated. In tropical regions, P. vivax relapses typically every 3-4 weeks, or every 6-8 weeks after treatment when individuals are treated with slow eliminating drugs which suppress the first relapse. In temperate areas, P. vivax can remain latent for 8-10 months between primary infection and first relapse (White N J., Malar J 2011; 10: e297; White N J et al., (2014) Lancet; 383: 723-35). In children, recurrent P. falciparum and P. vivax malaria have pronounced adverse effects as they interfere with growth, development, and schooling.
Malaria infections also have a strong impact on the shape of the human genome: The geographic distributions of sickle cell disease, haemoglobins C and E, ovalocytosis, thalassaemias, and glucose-6-phosphate dehydrogenase (G6PD) deficiency are roughly similar to that of malaria before the introduction of control measures, which suggests that these disorders confer a survival advantage in the presence of malaria. In the case of haemoglobin S [HbS], the heterozygote is protected against malaria, whereas the homozygote gets sickle cell disease. The adaptive genomic changes, which provide protective mechanisms against Malaria include decreased parasite growth at low oxygen tensions (haemoglobin AS [HbAS], reduced cytoadherence (haemoglobins AC [HbAC] and CC [HbCC], HbAS), reduced invasion (ovalocytosis), reduced parasite densities (G6PD defi ciency), and reduced multiplication at high densities (haemoglobin AE [HbAE]) (White N J et al., (2014) Lancet; 383: 723-35).
The primary objective in the treatment of Malaria is to ensure the rapid and complete elimination of the Plasmodium parasite from the patient's blood in order to prevent progression of uncomplicated malaria to severe disease or death, and to prevent chronic infection that leads to malaria-related anaemia. Furthermore, from a public health perspective, it is important to reduce transmission of the infection to others by means of treatment, by reducing the infectious reservoir, and to prevent the emergence and spread of resistance to antimalarial medicines.
The World Health Organization (WHO) recommends artemisinin-based combination therapies (ACTs) for the treatment of uncomplicated malaria caused by the P. falciparum parasite. By combining two active compounds with different mechanisms of action, ACTs are the most effective antimalarial medicines available today. The WHO currently recommends five ACTs for use against P. falciparum malaria. The choice of the respective ACT is based on the results of therapeutic efficacy studies against local strains of P. falciparum malaria.
In the treatment of P. falciparum malaria, artemisinin and its derivatives should not be used as oral monotherapy, as this promotes the development of artemisinin resistance in the parasite. Moreover, fixed-dose formulations, which combine two different active compounds co-formulated in one tablet are strongly preferred and recommended over co-blistered, co-packaged or loose tablet combinations, since they facilitate adherence to treatment and reduce the potential use of the individual components of co-blistered medicines as monotherapy. A treatment regimen of P. falciparum in children and adults typically comprises the administration of artesunate 2.4 mg/kg by intravenous or intramuscular injection, followed by 2.4 mg/kg at 12 h and 24 h with continued injections once daily if necessary. Where injectable treatment cannot be given, patients with severe malaria should immediately receive treatment with intrarectal artesunate and should be referred to an appropriate facility for full parenteral treatment.
The occurrence of malaria drug resistance is a major problem in the tropical and subtropical world, which is a phenomenon common to all anti-infective agents that can be defined as a genetically encoded reduction in efficacy of a drug. Anti-malarial drugs are amongst the most commonly used drugs worldwide. Historically, the administration of these drugs has been relatively unsupervised, which in combination with their frequency of use has led to the successive demise of drugs used first line treatments, such as chloroquine, proguanil, pyrimethamine, sulphadoxie-pyrimethamine and mefloquine. These drugs have become unable to produce a 90% clinical response in many areas where they have been intensively deployed (Ding et al. (2012) Malaria Journal 11:292).
Some anti-malarial medicines are more prone to resistance than others, e.g. resistant strains to chloroquine took decades to emerge, while for other drugs, such as atovaquone, which is an electron chain inhibitor, resistance emerged almost in parallel with its first clinical use. Studies have shown that the differences to build up a drug resistance has a molecular basis: chloroquine resistance requires several mutations in the transporter pfcrt (chloroquine resistance transporter), while resistance to atovaquone only requires a single point mutation in the mitochondrially-encoded cytochrome bcl pfcytb (cytochrome b) (Korsinczky M. et al., Antimicrob Agents Chemother (2000) 44:2100-2108). The emergence of resistance and its spread are influenced by several factors. Among them is the level of immunity against the malaria parasite in the population, e.g. in low or unstable transmission areas, drug resistance propagates rapidly. This is due to minimal immunity in a population, thus parasite infections lead to acute symptomatic disease which is most likely treated. Therefore, drug resistance is likely to propagate rapidly due to high drug pressure on existing parasites in these areas. In areas with a high level of disease immunity, the spread of drug resistance is restricted. Here, the requirement for treatment is reduced, due to fewer clinical symptoms in this population.
To date the development of resistance to arteminsinin has been relatively slow. This is partly due to the recommendation of the WHO that only fixed-dose combinations of arteminsinin derivatives with other anti-malarials should be used. However, the first signs of a reduction of the anti-parasitic activity of arteminsinins are emerging in Cambodia and Thailand, manifesting in a decrease in parasite clearance time.
Various anti-malarial drugs have been pursued to overcome malarial resistance and to provide novel and effective treatment options: Natural carbazol alkaloids have been used for the treatment of malaria in folklore medicine (Heterocycles, Vol 79, 2009, pages 121-144).
Calothrixins A and B have potential antimalarial effect (Tetrahedron 55 (1999) 13513-13520). Carbazol derivatives have been synthesized to inhibit the Plasmodium falciparum pyrimidine biosynthetic enzyme (J. Med. Chem., 2007, 50, 186-191). Other carbazole derivatives have been disclosed in WO0129028, WO2010/010027, WO2007/062399, WO2005/074971 and WO02/060867.
Despite of countless efforts to control malaria and its further spread, there is a continued requirement for novel and effective anti-malarial drugs that overcome the emerging resistance to currently available anti-malarial drugs.
It is thus an objective of the present invention to provide novel and effective compounds, for use in the treatment of parasitic diseases such as malaria.